Objective lens for pickup and light pickup apparatus

ABSTRACT

An objective lens for use in an optical pickup apparatus to record or reproduce information in an optical information recording medium, comprises an aspheric surface, wherein the following conditional formula is satisfied: 1.1≦d 1 /f≦3 where d 1  represents axial lens thickness and f represents a focal length.

This application is a divisional of U.S. patent application Ser. No.09/653,942, filed Sep. 1, 2000, now U.S. Pat. No. 6,411,442 which ishereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to an objective lens for pickup in anoptical recording apparatus which conducts recording on or reproducingfrom an information recording medium such as an optical disk, and to aoptical pickup apparatus employing the objective lens.

In an optical system of an optical recording/reproducing apparatusemploying a medium of an optical disk, there is commonly used anaspherical single objective lens. For achieving high density of recordedinformation signals, a size of a spot formed on a recording medium bythe objective lens has been required to be small, and there have beeninvestigated high NA of an objective lens and utilization of a lightsource for short wavelength.

Though there has been developed a GaN blue semiconductor laser having anoscillation wavelength of about 400 nm, a wavelength is varied by modehop or by laser output, and monochromaticity of oscillation wavelengthis poor because high-frequency superimposition is used. Therefore, in alight-converging optical system for high density optical disk wherein aGaN blue semiconductor laser is used, it is considered that correctionof axial chromatic aberration is necessary.

In an aspherical single objective lens for an optical disk, sphericalaberration and coma are corrected by aspherical surfaces. However, whena numerical aperture is large, image height characteristics aredeteriorated. When an optical disk is made to be of higher density,deterioration of the image height characteristics becomes an extremelyserious problem even if the value of the deterioration is small. Inparticular, when the numerical aperture is 0.65 or more, the problem isconspicuous. Further, when the numerical aperture is greater,deterioration of eccentricity sensitivity is also a serious problem.

SUMMARY OF THE INVENTION

The invention has been achieved for solving the problem stated above.Namely, with regard to an objective lens for a optical pickup apparatus,an object of the invention is to provide an aspherical single objectivelens whose numerical aperture is great and image height characteristicsare excellent. In particular, an object of the invention is to providean objective lens which is suitable to be used for a high densityrecording/reproducing apparatus wherein a numerical aperture is not lessthan 0.65, preferably 0.7 or more, and more preferably 0.75 or more, andthere is used a laser wherein a wavelength of a light source is as shortas about 500 nm.

Further, providing an objective lens which makes eccentricitysensitivity to be excellent is also an object of the invention.Furthermore, providing an objective lens which makes sphericalaberration and coma to be excellent is also an object of the invention.

When a thickness of a protective layer (transparent base board) of aninformation recording medium is small to be 0.2 mm or less, or whenthere is no protective layer, a working distance can be small. An offerof an objective lens which is suitable to be used in arecording/reproducing apparatus having such small working distance is anobject of the invention.

It is further an object of the invention to provide a optical pickupapparatus, an optical information recording medium recording/reproducingapparatus, and an optical information recording. mediumrecording/reproducing method, which employ these objective lenses statedabove.

Further, an object is to provide a optical pickup apparatus having anoptical system wherein axial chromatic aberration is corrected by thesimple structure, in a high density optical recording/reproducingapparatus. In particular, it is an object to provide a optical pickupapparatus wherein a numerical aperture on the part of an informationrecording medium is 0.65 or more, preferably 0.7 or more, and morepreferably 0.75 or more, and shortest wavelength of a light source to beused is as small as 500 nm or less.

The above object can be attained by the following structures.

(1) An objective lens for use in an optical pickup apparatus to recordor reproduce information in an optical information recording medium,comprising:

an aspheric surface;

wherein the following conditional formula is satisfied:

 1.1≦d 1/f≦3

 where d1 represents axial lens thickness and f represents a focallength.

(2) In the objective lens of (1), a numerical aperture of the objectivelens is not smaller than 0.65.

(3) In the objective lens of (2), a numerical aperture of the objectivelens is not smaller than 0.75.

(4) In the objective lens of (1), the following conditional formula issatisfied:

f/vd≦0.060

 where vd represents Abbe's number.

(5) In the objective lens of (1), the following conditional formula issatisfied:

1.40≦n

 where n represents a refractive index at a used wavelength.

(6) In the objective lens of (5), the following conditional formula issatisfied:

1.40≦n≦1.85

(7) In the objective lens of (1), the following conditional formula issatisfied:

0.40≦r 1/(n·f)≦0.70

 where r1 represents a paraxial radius of curvature of one surface ofthe objective lens.

(8) In the objective lens of (7), r1 represents a paraxial radius ofcurvature of the surface of the objective lens at the light source side.

(9) In the objective lens of (1), a used wavelength is not longer than500 nm.

(10) In the objective lens of (1), the objective lens is an objectivelens for use in an optical pickup apparatus to record or reproduceinformation in an optical information recording medium having aprotective layer whose thickness is not larger than 0.2 mm.

(11) In the objective lens of (10), a numerical aperture is not smallerthan 0.7.

(12) In the objective lens of (7), the following conditional formula issatisfied:

1.50≦n

 where n represents a refractive index at a used wavelength.

(13) In the objective lens of (1), the objective lens is a plastic lens.

(14) In the objective lens of (1), the objective lens is a glass lens.

(15) In the objective lens of (1), the following conditional formula issatisfied:

1.85≦n

 where n represents a refractive index at a used wavelength.

(16) In the objective lens of (1), the objective lens further comprisesa diffracting section.

(17) In the objective lens of (1), the objective lens further comprisesa flange section on an outer periphery thereof.

(18) In the objective lens of (1), the flange section comprises asurface extended in a direction perpendicular to an optical axis.

(19) In the objective lens of (1), each of both lens surfaces is anaspherical surface.

(20) An optical pickup apparatus to record or reproduce information inan optical information recording medium, comprises:

a light source to emit light flux;

a converging optical system to condense the light flux emitted from thelight source; and

an optical detector to detect reflection light from the opticalinformation recording medium;

 wherein the converging optical system comprises an objective lens tocondense the light flux on an information recording surface of theoptical information recording medium and the objective lens comprises anaspheric surface; and

wherein the following conditional formula is satisfied:

1.1≦d 1/f≦3

 where d1 represents axial lens thickness of the objective lens and frepresents a focal length of the objective lens.

(21) In the optical pickup apparatus of (20), a numerical aperture ofthe objective lens at the optical information recording medium side isnot smaller than 0.65.

(22) In the optical pickup apparatus of (29), a numerical aperture ofthe objective lens at the optical information recording medium side isnot smaller than 0.75.

(23) In the optical pickup apparatus of (20), the following conditionalformula is satisfied:

f/vd≦0.060

 where vd represents Abbe's number of the material of the objectivelens.

(24) In the optical pickup apparatus of (20), the following conditionalformula is satisfied:

1.40≦n

 where n represents a refractive index of the material of the objectivelens at a used wavelength.

(25) In the optical pickup apparatus of (24), the following conditionalformula is satisfied:

1.40≦n≦1.85

(26) In the optical pickup apparatus of (20), the following conditionalformula is satisfied:

 0.40≦r 1/(n·f)≦0.70

 where r1 represents a paraxial radius of curvature of the surface ofthe objective lens at the light source side.

(27) In the optical pickup apparatus of (20), the light source emitslight flux whose wavelength is not larger than 500 nm.

(28) In the optical pickup apparatus of (20), the optical pickupapparatus is used to record or reproduce information in an opticalinformation recording medium having a protective layer whose thicknessis not larger than 0.2 mm.

(29) In the optical pickup apparatus of (28), a numerical aperture ofthe objective lens at the optical information recording medium side isnot smaller than 0.7.

(30) In the optical pickup apparatus of (20), the following conditionalformula is satisfied:

1.85≦n

 where n represents a refractive index of a material of the objectivelens at a wavelength of the light flux emitted from the light source.

(31) In the optical pickup apparatus of (20), the converging opticalsystem comprises a diffracting section.

(32) In the optical pickup apparatus of (20), the converging opticalsystem comprises a coupling lens to change a divergent angle of thelight flux emitted from the light source and the coupling lens correctschromatic aberration of the objective lens.

(33) In the optical pickup apparatus of (32), the coupling lens is acollimator lens to make the light flux emitted from the light source tobe parallel light flux.

(34) In the optical pickup apparatus of (32), chromatic aberration ofthe composite system of the objective lens and the coupling lenssatisfies the following conditional formula:

δfb·NA ²≦0.25 μm (δfb>0)

 where δfb represents a change of focal position (μm) of the compositesystem when a wavelength is changed from a standard wavelength by +1 nm,and NA represents a numerical aperture of the objective lens at theoptical information recording medium side.

(35) In the optical pickup apparatus of (34), the chromatic aberrationof the composite system of the objective lens and the coupling lenssatisfies the following conditional formula:

0.02 μm≦δfb·NA ²≦0.15 μm (δfb>0)

(36) In thee optical pickup apparatus of (32), the following conditionalformula is satisfied:

0.1≦|m|≦0.5 (m≦0)

 where m represents magnification of the composite system of theobjective lens and the coupling lens.

(37) In the optical pickup apparatus of (32), the coupling lens is asingle lens group having two lenses.

(38) In the optical pickup apparatus of (32), the coupling lenscomprises an aspherical surface.

(39) In the optical pickup apparatus of (32), the coupling lenscomprises a diffracting section.

(40) An apparatus to record or reproduce information in an opticalinformation recording medium, comprises:

an optical pickup apparatus, comprising

a light source to emit light flux;

a converging optical system to condense the light flux emitted from thelight source; and

an optical detector to detect reflection light or transmission lightfrom the optical information recording medium;

 wherein the converging optical system comprises an objective lens tocondense the light flux on an information recording surface of theoptical information recording medium and the objective lens comprises anaspheric surface; and

wherein the following conditional formula is satisfied:

1.1≦d 1/f≦3

 where d1 represents axial lens thickness of the objection lens and frepresents a focal length of the objective lens.

(41) A method of recording or reproducing information in an opticalinformation recording medium, comprises:

a step of emitting light flux;

a step of converging the light flux emitted from the light source on aninformation recording surface of the optical information recordingmedium; and

a step of detecting reflection light or transmission light of the lightcondensed on the information recording surface;

 wherein the light flux is converged on the information recordingsurface of the optical information recording medium by an objectivelens; and

wherein the objective lens comprises an aspheric surface and thefollowing conditional formula is. satisfied:

1.1≦d 1/f≦3

 where d1 represents axial lens thickness of the objective lens and frepresents a focal length of the objective lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows a sectional view of an objective lens and FIG. 1(b)shows aberration diagrams of the objective lens in Example 1.

FIG. 2(a) shows a sectional view of an objective lens and FIG. 2(b)shows aberration diagrams of the objective lens in Example 2.

FIG. 3(a) shows a sectional view of an objective lens and FIG. 3(b)shows aberration diagrams of the objective lens in Example 3.

FIG. 4(a) shows a sectional view of an objective lens and FIG. 4(b)shows aberration diagrams of the objective lens in Example 4.

FIG. 5(a) shows a sectional view of an objective lens and FIG. 5(b)shows aberration diagrams of the objective lens in Example 5.

FIG. 6(a) shows a sectional view of an objective lens and FIG. 6(b)shows aberration diagrams of the objective lens in Example 6.

FIG. 7(a) shows a sectional view of an objective lens and FIG. 7(b)shows aberration diagrams of the objective lens in Example 7.

FIG. 8 is a diagram showing an embodiment of a optical pickup apparatusemploying an objective lens of the invention.

FIG. 9 shows a sectional view of an objective lens in Example 8.

FIG. 10 shows aberration diagrams of the objective lens in Example 8.

FIG. 11 shows a sectional view of an objective lens in Example 9.

FIG. 12 shows aberration diagrams of the objective lens in Example 9.

FIG. 13 shows a sectional view of an objective lens in Example 10.

FIG. 14 shows aberration diagrams of the objective lens in Example 10.

FIG. 15 shows a sectional view of an objective lens in Example 11.

FIG. 16 shows aberration diagrams of the objective lens in Example 11.

FIG. 17 shows a sectional view of an objective lens in Example 12.

FIG. 18 shows aberration diagrams of the objective lens in Example 12.

FIG. 19 shows a sectional view of an objective lens in Example 13.

FIG. 20 shows aberration diagrams of the objective lens in Example 13.

FIG. 21 shows a sectional view of an objective lens in Example 14.

FIG. 22 shows aberration diagrams of the objective lens in Example 14.

FIG. 23 shows a sectional view of an objective lens in Example 15.

FIG. 24 shows aberration diagrams of the objective lens in Example 15.

FIG. 25 shows sectional views of a coupling lens and an objective lensin Example 16.

FIG. 26 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 16.

FIG. 27 shows sectional views of a coupling lens and an objective lensin Example 17.

FIG. 28 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 17.

FIG. 29 shows sectional views of a coupling lens and an objective lensin Example 18.

FIG. 30 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 18.

FIG. 31 shows sectional views of a coupling lens and an objective lensin Example 19.

FIG. 32 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 19.

FIG. 33 shows sectional views of a coupling lens and an objective lensin Example 20.

FIG. 34 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 20.

FIG. 35 shows sectional views of a coupling lens and an objective lensin Example 21.

FIG. 36 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 21.

FIG. 37 shows sectional views of a coupling lens and an objective lensin Example 22.

FIG. 38 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 22.

FIG. 39 shows sectional views of a coupling lens and an objective lensin Example 23.

FIG. 40 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 23.

FIG. 41 shows sectional views of a coupling lens and an objective lensin Example 24.

FIG. 42 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 24.

FIG. 43 shows sectional views of a coupling lens and an objective lensin Example 25.

FIG. 44 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 25.

FIG. 45 shows sectional views of a coupling lens and an objective lensin Example 26.

FIG. 46 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 26.

FIG. 47 shows sectional views of a coupling lens and an objective lensin Example 27.

FIG. 48 shows spherical aberration diagrams of the coupling lens and theobjective lens in Example 28.

FIG. 49 shows sectional views of an objective lens in Example 28.

FIG. 50 shows aberration diagrams of the objective lens in Example 28.

FIG. 51 shows sectional views of an optical system in Example 29.

FIG. 52 shows an aberration diagram of the optical system in Example 29.

FIG. 53 shows sectional views of an optical system in Example 30.

FIG. 54 shows an aberration diagram of the optical system in Example 30.

FIG. 55 shows sectional views of an optical system in Example 31.

FIG. 56 shows an aberration diagram of the optical system in Example 31.

FIG. 57 is a diagram showing another embodiment of a optical pickupapparatus employing an objective lens of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention will be explained as follows.

An aspherical single objective lens according to the first invention isan objective lens for recording on and reproducing from an informationrecording medium, and it is characterized to satisfy the followingexpression.

Incidentally, an objective lens of the invention has at least oneaspherical surface. It is preferable that both sides of the objectivelens represent an aspherical surface, although it is allowable that oneside only is made to be an aspherical surface. It is further preferablethat an objective lens is composed of one piece of lens without beingcomposed of plural pieces of lenses.

1.1≦d 1/f≦3  (1)

wherein d1 represents an axial lens thickness, and f represents a focallength.

The conditional expression (1) above represents conditions for obtainingexcellent image height characteristics, and when trying to obtain agreat numerical aperture which is not less than 0.65, or not less than0.75 preferably, in particular, if a value of d1/f is not less than thelower limit, a central thickness of a lens is not too small, imageheight characteristics are not deteriorated, and further, shiftsensitivity does not grow greater. If a value of d1/f is not more thanthe upper limit, the central thickness is not too large, and imageheight characteristics are not deteriorated. Incidentally, it ispreferable that d1 is in a range of 2 mm-4 mm.

Further, eccentricity sensitivity becomes excellent. In addition,spherical aberration and coma can be corrected satisfactorily. A opticalpickup apparatus which reproduces or records information on an opticalinformation recording medium of the invention has therein a light sourcewhich emits a light flux, a light converging optical system whichconverges a light flux emitted from the light source and a lightdetection unit which detects a reflected light or a transmitted lightcoming from an optical information recording medium. The lightconverging optical system has an objective lens which converges a lightflux on an information recording surface of an optical informationrecording medium. The objective lens is one in the invention statedabove. Incidentally, the light converging optical system may also have acoupling lens in addition to the objective lens. The optical informationrecording medium recording/reproducing apparatus of the invention hasthe optical pickup apparatus of the invention stated above. In additionto this, the optical information recording medium recording/reproducingapparatus may also have a spindle motor which rotates an opticalinformation recording medium and a tracking means. Incidentally, it ispreferable that a numerical aperture in the optical pickup apparatus isobtained based on a wavelength of a light source, a diameter of anaperture and a diameter of an objective lens. Incidentally, in theoptical pickup apparatus, a numerical aperture which makes it possiblefor a light flux with prescribed wavelength to read/record informationon prescribed optical information recording medium can be taken as anumerical aperture of the optical pickup apparatus, or a numericalaperture established by a standard of the optical information recordingmedium to be read/recorded by the optical pickup apparatus can be takenas a numerical aperture of the optical pickup apparatus. Further, whenjudging a numerical aperture from only a lens, if the lens is correctedto have no aberration for a range within a certain radius of an apertureof the lens (for example, the wave front aberration is corrected to be0.07 λ or less), the numerical aperture may be defined as a ratio ofthis radius to the focal length.

It is more desirable that the conditional expression (1) above satisfiesthe following expression.

1.2≦d 1/f≦2.3

Further, it is most desirable that the conditional expression (1) abovesatisfies the following expression.

1.5≦d 1/f≦1.8

Further, it is preferable that the objective lens stated above satisfiesthe following conditional expression (2);

f/vd≦0.060  (2)

wherein vd represents Abbe's number.

The conditional expression (2) above represents conditions to make axialchromatic aberration small. Due to this, it is possible to cope withinstantaneous wavelength fluctuations in a laser light source which cannot be followed by a servomechanism for focusing, and to cope withextension of wavelength in a light source caused by multi-modeoscillation. It is desirable that the conditional expression (2) abovesatisfies the following expression.

f/vd≦0.050

Further, it is most desirable that the conditional expression (2) abovesatisfies the following expression.

f/vd≦0.035

With regard to a lens material, it is preferable that the materialwherein Abbe's number preferably satisfies vd=50 rather than vd=35 isused.

The objective lens may be either a glass lens or a plastic lens, but theplastic lens is more preferable. When the objective lens is a plasticlens, it is preferable that saturation water absorption of lens plasticis not more than 0.01%. Further, a material whose light transmission forlight with wavelength of 350 nm-500 nm is not less than 85% ispreferable. It is preferable that a diameter of the objective lens ofthe invention is 2.0 mm to 4.0 mm. As a material for a plastic lens,polyolefine resins are preferable. In particular, norbornene resins arepreferable.

Further, it is preferable that the objective lens stated above satisfiesthe following conditional expression (3);

1.40≦n  (3)

wherein, n represents a refractive index (refractive index of a materialof the objective lens at the wavelength of the light source) at thewavelength used.

The conditional expression (3) above represents a condition of arefractive index, and when this condition is satisfied and a refractiveindex is not made small, a sag on the first surface does not growgreater, shift sensitivity and tilt sensitivity on the surface do notgrow greater, and image height characteristics are not deteriorated.

Further, it is preferable that the objective lens stated above satisfiesthe following conditional expression (4).

1.40≦n≦1.85  (4)

The conditional expression (4) above represents a condition of arefractive index, and in the case of a optical pickup wherein greatimportance needs to be attached not only to axial optical power but alsoto off-axial optical power, an axial thickness tends to be greaterbecause of correction of astigmatism generated. When n is not more thanthe upper limit, a refractive index is not made to be too great and thecentral thickness of a lens does not need to be made great, which makesit easy to attain the light weight and to ensure a working distance.When n is not less than the lower limit, a refractive index is not madeto be too small, a sag on the first surface does not grow greater, andimage height characteristics are not deteriorated.

It is desirable that the conditional expression (4) above satisfies thefollowing expression.

1.50≦n≦1.85

The more preferable is to satisfy the following expression.

 1.70≦n≦1.85

Further, it is preferable that the objective lens stated above satisfiesthe following conditional expression (5);

0.40≦r 1/(n·f)≦0.70  (5)

wherein, r1 represents a paraxial radius of curvature of one surface ofthe objective lens stated above (preferably, a paraxial radius ofcurvature on the part of a light source).

The conditional expression (5) above relates mainly to correction ofcoma, and when a value of r1/(n·f) is not less than the lower limit, r1is not made to be too small, and a flare caused by introversive coma andextroversive coma becomes hard to be generated, while, when a value ofr1/(n·f) is not more than the upper limit, r1 is not made to be toolarge, extroversive coma is hard to be generated, and an under flare ofspherical aberration is hard to be generated.

It is desirable that the conditional expression (5) above satisfies thefollowing expression.

0.40≦r 1/(n·f)≦0.65

A diameter of a spot of light converged on a recording medium by anobjective lens is generally determined by kλ/NA when λ represents awavelength of a light source, NA represents a numerical aperture of theobjective lens, and k represents a proportional constant. Therefore,when a laser light source with a small wavelength of 500 nm or less isused and a numerical aperture of the objective lens is made to be aslarge as 0.65 or more, it is possible to make a diameter of a spot oflight to be converged to be small. It is therefore possible to makerecorded information signals to be of high density, by constituting aoptical pickup apparatus by the use of a lens related to the invention.Further, it is possible to provide an objective lens having a smallworking distance by making a protective layer of a recording medium tobe as thin as 0.2 mm or less, which makes it possible to attain thelight weight and compactness of a optical pickup apparatus.

In other words, the objective lens, the optical pickup apparatus and theoptical information recording medium recording/reproducing apparatus,all in the invention are especially suitable when a wavelength used(wavelength of a light flux emitted from a light source) is not morethan 500 nm, or when the numerical aperture of the objective lens on theoptical information recording medium side is not less than 0.65(preferably, not less than 0.7, more preferably not less than 0.75), orwhen they are used for an optical information recording medium having aprotective layer with a thickness of 0.2 mm or less.

When the objective lens mentioned above is made of plastic materials, itis possible to attain the light weight of a optical pickup apparatus,and to realize mass production at low cost.

Another preferable aspherical single objective lens is an objective lensfor recording on and reproducing from an information recording medium,and the objective lens is characterized in that a wavelength to be usedis 500 nm or less, an information recording medium has a protectivelayer having a thickness of 0.2 mm or less, and a numerical aperture ofthe objective lens is 0.65 or more, and preferably is 0.75 or more.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (6) representing conditions forobtaining excellent image height characteristics wherein operationsthereof are the same as those in conditional expression (1);

1.1≦d 1/f≦3  (6)

wherein, d1 represents axial lens thickness and f represents a focallength.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (7) representing conditions for makingaxial chromatic aberration wherein operations thereof are the same asthose in conditional expression (2);

f/vd≦0.060  (7)

wherein, vd represents Abbe's number.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (8) representing conditions for arefractive index wherein operations thereof are the same as those inconditional expression (3);

1.40≦n  (8)

wherein, n represents a refractive index at the wavelength used.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (9). The conditional expression (9)represents conditions for a refractive index. Operations thereof are thesame as those in conditional expression (4).

1.40≦n<1.85  (9)

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (10) relating mainly to correction ofcoma wherein operations thereof are the same as those in conditionalexpression (5);

0.40≦r 1/(n·f)≦0.70  (10)

wherein, r1 represents paraxial radius of curvature on the part of alight source.

Another preferable aspherical single objective lens is an objective lensfor recording on and reproducing from an information recording medium,and the objective lens is characterized in that its numerical apertureis 0.75 or more and it is made of plastic materials.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (11) representing conditions forobtaining excellent image height characteristics wherein operationsthereof are the same as those in conditional expression (1);

1.1≦d 1/f≦3  (11)

wherein, d1 represents axial lens thickness, and f represents a focallength.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (12) representing conditions for makingaxial chromatic aberration to be small wherein operations thereof arethe same as those in conditional expression (2);

f/vd≦0.060  (12)

wherein, vd represents Abbe's number.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (13). representing conditions for arefractive index wherein operations thereof are the same as those inconditional expression (3);

1.40≦n  (13)

wherein, n represents a refractive index at the wavelength used.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (14). The conditional expression (14)represents conditions for a refractive index. Operations thereof are thesame as those in conditional expression (4).

1.40≦n≦1.85  (14)

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (15) relating to correction of comawherein operations thereof are the same as those in conditionalexpression (5);

0.40≦r 1/(n·f)≦0.70  (15)

wherein, r1 represents paraxial radius of curvature on the part of alight source.

Another preferable aspherical single objective lens is an objective lensfor recording on and reproducing from an information recording medium,and the objective lens is characterized in that its numerical apertureis 0.65, and preferably is 0.75 or more and it satisfies the followingexpression (16);

n≧1.85  (16)

wherein, n represents a refractive index at the wavelength used.

The conditional expression (16) above represents conditions of arefractive index. By using materials of high refractive index, it ispossible to make a radius of curvature on the first surface to be large,and as a result, it is possible to make a prospective angle to be small.Thus, there is a merit that it is easy to process a metal mold whenmaking a lens through molding. Further, in the case of a optical pickupwherein great importance needs to be attached only to axial opticalpower, the use of materials of high refractive index makes it easy tocorrect spherical aberration of a high order.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (17) representing conditions forobtaining excellent image height characteristics wherein operationsthereof are the same as those in conditional expression (1);

1.1≦d 1/f≦3  (17)

wherein, d1 represents axial lens thickness and f represents a focallength.

It is preferable that the aforesaid objective lens satisfies thefollowing conditional expression (18) relating to correction of comawherein operations thereof are the same as those in conditionalexpression (5);

0.40≦r 1/(n·f)≦0.70  (18)

wherein, r1 represents paraxial radius of curvature on the part of alight source.

Another aspherical single objective lens is an objective lens forrecording on and reproducing from an information recording medium, andthe objective lens is characterized in that its numerical aperture is0.65, and preferably is 0.75 or more and it satisfies the followingexpression (19);

1.40≦n≦1.85  (19)

wherein, n represents a refractive index at the wavelength used.

The aforesaid conditional expression represents conditions of arefractive index. In respect to its operations, they are the same asthose in expression (4).

It is preferable to provide a diffraction section on a light convergingoptical system in the optical pickup apparatus of the invention. Thoughit is preferable to provide a diffraction section on an objective lensof the light converging optical system, it is possible either toincorporate an optical element having only a diffraction section in thelight converging optical system or to provide a diffraction section onanother optical element which constitutes a light converging opticalsystem such as a coupling lens. Incidentally, it is also possible toprovide a diffraction surface only on one side of a single couplinglens. Owing to this structure, it is possible to prevent deteriorationof wavefront aberration caused by surface eccentricity.

When spherical aberration is corrected for the standard wavelength byonly a spherical surface or by only an aspherical surface, for a singleobjective lens in a refraction system, there is normally generated underaxial chromatic aberration for the wavelength that is shorter than thestandard wavelength, and over axial chromatic aberration for thewavelength that is longer than the standard wavelength. However, in thecase of the objective lens having a diffraction surface, when sphericalaberration is corrected for the standard wavelength, it is possible togenerate polarity which is opposite to the objective lens in arefraction system, namely, to generate over axial chromatic aberrationfor the wavelength that is shorter than the standard wavelength andunder axial chromatic aberration for the wavelength that is longer thanthe standard wavelength. Therefore, in the case of the objective lensstated above, it is possible to realize an objective lens which showsexcellent performance even for instantaneous wavelength fluctuationssuch as mode hop, for example, by combining diffraction powers throughappropriate selection of the aspheric surface coefficient as anaspherical lens and a coefficient of a phase function as a diffractionlens, and thereby by correcting chromatic aberration for the sphericalaberration.

The objective lens stated above has a numerical aperture of 0.65 ormore, or preferably of 0.75 or more, and when a wavelength to be used is500 nm or less, and when an information recording medium wherein athickness of a protective layer is 0.2 mm or less is used, it ispossible to use a laser light source having a short wavelength of notmore than 500 nm, to make a numerical aperture of the objective lens tobe as large as 0.65 or more, and to make a spot diameter of light to beconverged to be small. Thus, it is possible to realize high density ofrecorded information signals, by constituting a optical pickup apparatuswith a lens of the invention. Further, it is possible to provide anobjective lens having a small working distance, by making a protectivelayer of a recording medium to be as thin as 0.2 mm or less, which makesit possible to attain the light weight and compactness of a opticalpickup apparatus.

Further, in each objective lens mentioned above, it is preferable that aflange portion is provided on the outer circumference, and it is morepreferable that there is provided on the outer circumference a flangeportion having a surface extending in the direction perpendicular to anoptical axis. Due to the flange portion provided on the outercircumference, the objective lens can be attached on the optical pickupapparatus easily, and it is possible to attach the objective lens moreaccurately by providing on the flange portion a surface extending in thedirection almost perpendicular to an optical axis.

Further, each optical pickup apparatus of the invention is one havingtherein a light source and an objective lens which converges a lightflux from the light source on the information recording surface of aninformation recording medium, and conducting information recording onthe information recording medium and/or information reproducingtherefrom, through detection of light from the information recordingmedium, wherein an aspherical single objective lens is provided as theobjective lens mentioned above.

Further, each optical pickup apparatus of the invention is one havingtherein a light source, a coupling lens which changes a divergence angleof a divergent light emitted from the light source, and an objectivelens which converges a light flux through the coupling lens on theinformation recording surface of an information recording medium, andconducting information recording on the information recording mediumand/or information reproducing therefrom, through detection of lightfrom the information recording medium, wherein the coupling lens hasfunctions to correct chromatic aberration of the objective lens, and theaspherical single objective lens is provided as the objective lensmentioned above.

When an aspherical single objective lens is used as an objective lens,it is possible to obtain the objective lens suitable for a high densityrecording/reproducing apparatus but there is caused axial chromaticaberration which is under on the part of a short wavelength, because ofthe single objective lens in a refraction system. However, the axialchromatic aberration can be corrected by the coupling lens in theaforesaid optical pickup apparatus. Namely, if axial chromaticaberration of the coupling lens is made to be over on the part of ashort wavelength, axial chromatic aberration of the objective lens canbe reduced. Due to this, together with the objective lens mentionedabove, it is possible to obtain a optical pickup apparatus having anoptical system wherein axial chromatic aberration is corrected by thesimple structure.

In this case, the coupling lens can collimate a light flux emitted fromthe light source to the mostly collimated light flux. This structuremakes assembly and adjustment of a pickup optical system to be simple.Namely, the coupling lens may also be a collimator lens.

It is further possible to arrange so that chromatic aberration of acomposition system of the objective lens and the coupling lens maysatisfy the following expression (20);

δfb·NA ²≦0.25 μm (δfb>0)  (20)

wherein δfb represents a change of focal position (μm) of thecomposition system when a wavelength is changed from the standardwavelength by +1 nm, and NA represents a numerical aperture of theobjective lens on the part of a disk.

It is more preferable that the following expression (20)′ is satisfied.

0.02 μm≦δfb·NA ²≦0.15 μm (δfb>0)  (20)′

Each structure stated above is one relating to the correction ofchromatic aberration carried out by the coupling lens. When handling ashort wavelength laser semiconductor with oscillation wavelength ofabout 400 nm, axial chromatic aberration caused on the objective lens bythe microscopic shift of wavelength is an unallowable serious problem.The causes for the problem are given as follows. When handling a shortwavelength, a change in refractive index for ordinary lens materials islarge for microscopic fluctuation of a wavelength. As a result, adefocusing amount for the focus is large. With regard to a focal depthof the objective lens, however, the shorter the wavelength (λ) to beused is, the smaller the focal depth is, as is understood from kλ/NA² (kis a proportional constant), and even a very small amount of defocusingis not allowed. In Session WD26 of ISOM/ODS'99 Postdeadline PosterPapers, high frequency superimposition with spectrum width of 0.7 nm(FWMH) is shown for GaN blue semiconductor laser. It is desirable tocontrol wave front aberration of a pickup optical system to about 0.02λrms for the high frequency superimposition. A level of correction ofaxial chromatic aberration necessary for the foregoing was obtained, onthe assumption that spherical aberration for color has been corrected.Whereupon, it was necessary to control the axial chromatic aberration ofthe composition system within about 0.15 μm/NA² for wavelengthfluctuation of 1 nm, for controlling wave front aberration to 0.02 λrmsfor the high frequency superimposition with spectrum width of 0.7 nm(FWMH), when the standard wavelength is 400 nm and NA represents anumerical aperture on the part of a disk. On the other hand, the axialchromatic aberration of the composition system does not always need tobe corrected perfectly, and wave front aberration can remain in anallowable range. When the objective lens is a single lens in arefraction system as in the invention, it is possible to constitute acoupling lens in a simple way by leaving the axial chromatic aberrationto be a positive value for a long wavelength even in the compositionsystem, because the axial chromatic aberration is of a positive valuefor the long wavelength for the objective lens. For example, when acoupling lens is composed of a one-group and two-element cemented lens,power of each lens element of the coupling lens can be weak, comparedwith perfect color correction of the composition system, resulting in acoupling lens which is excellent in terms of aberration and is easy tomake. Even in the case of correcting color for a coupling lens as adiffraction lens, power of a diffraction surface can be weak, thus, aninterval of zonal diffraction rings is large, and a diffraction lenswith high diffraction efficiency can easily be manufactured. For thisreason, the lower limit of the aforesaid conditional expression wasestablished.

Further, it is preferable to satisfy the following expression concerningmagnification m of a composition system;

0.1≦|m|≦0.5  (m≦0)

wherein, m represents magnification of a composition system of anobjective lens and a coupling lens.

When the magnification is not lower than the lower limit of theconditional expression above, the composition system is compact, while,when the magnification is not higher than the upper limit, the couplinglens is better in terms of aberration.

The coupling lens may be composed either of one piece or of pluralpieces, and it is preferable that the coupling lens is of the one-groupand two-element structure. Due to the one-group and two-elementstructure of the coupling lens stated above, the structure of thecoupling lens is simple and it causes manufacture of the coupling lensto be easy. When using a one-group and two-element coupling lens, it ispossible to generate sharply the axial chromatic aberration which isover on the part of a short wavelength and is under on the part of along wavelength, while keeping the axial power. As a result, it ispossible to correct favorably the axial chromatic aberration on theobjective lens which is under on the part of a short wavelength and isover on the part of a long wavelength, while keeping the axial power onthe composition system, which is advantageous for instantaneouswavelength fluctuations such as mode hop. When this axial chromaticaberration is made to be over on the part of a short wavelength and tobe under on the part of a long wavelength, the curvature on the cementedsurface of the coupling lens having diverging actions tends to be great.Therefore, if spherical aberration at the standard wavelength iscontrolled, spherical aberration which is over on the part of a shortwavelength and is under on the part of a long wavelength is generatedgreatly. As a result, spherical aberrations on the part of a shortwavelength and on the part of a long wavelength generated on theobjective lens are canceled, and spherical aberration of a compositionsystem in the case of wavelength fluctuations cab be controlled to besmall.

Incidentally, it is preferable that the coupling lens has an asphericalsurface. The aspherical surface can be provided either on one side or onboth sides.

Since the coupling lens stated above is of a one-group and two-elementstructure having an aspheric surface, it is possible to make a numericalaperture of the coupling lens to be large by aberration correctionfunctions of the aspheric surface and to obtain a compact compositionsystem whose total length is short.

Since the coupling lens stated above has a diffraction surface, it ispossible to obtain a highly efficient coupling lens with a simplestructure of a single lens, by adding, in particular, a diffractionsurface to a plastic aspherical lens. Incidentally, it is also possibleto correct fluctuations of spherical aberration caused on each opticalsurface of an optical system, by moving a coupling lens in the opticalaxis direction. For example, a coupling lens may be moved whilemonitoring RF amplitude of a reproduction signal, so that sphericalaberration caused in the optical system may be corrected in an optimalway. As fluctuations of spherical aberration caused on each opticalsurface of an optical system, there are given, as an example, thefluctuation based on minute changes in oscillation wavelength of a lightsource, the fluctuation based on temperature change, the fluctuationbased on humidity change, the fluctuation based on minute change in athickness of a protective layer of an information recording medium andthe fluctuation based on combination of the foregoing. It is preferablethat a coupling lens is shifted in the optical axis direction so that adistance from an objective lens may be increased when sphericalaberration of an optical system fluctuates to the greater side, and acoupling lens is shifted in the optical axis direction so that adistance from an objective lens may be decreased when sphericalaberration of an optical system fluctuates to the smaller side.Incidentally, with regard to a movement of the coupling lens in theoptical axis direction, it is preferable that a optical pickup apparatushas a shifting unit which shifts the coupling lens. As a shifting unit,a voice-oil-shaped actuator and a piezo-actuator can be used.

Incidentally, each optical pickup apparatus mentioned above converges alight flux emitted from a laser light source through the objective lenson the information recording surface of an information recording medium,and it can conduct recording of information on the information recordingmedium and/or reproducing of information from the information recordingmedium.

The optical information recording medium stated above includes, forexample, various CDs such as CD, CD-R, CD-RW, CD-Video and CD-ROM,various DVD such as DVD, DVD-ROM, DVD-RAM, DVD-R and DVD-RW, and adisk-shaped information recording medium such as MD, and further, anovel high density information recording medium that has been enhancedin terms of recording density is included.

Embodiments of the invention will be explained as follows, referring tothe drawings. FIG. 8 is a schematic structure diagram of a opticalpickup apparatus showing an embodiment of the invention.

A optical pickup apparatus in FIG. 8 is one employing the doubleaspherical single objective lens of the invention as an objective lens,wherein semiconductor laser 3 representing a light source, coupling lens2 which changes a divergence angle of a divergent light emitted from thelight source 3, objective lens 1 which converges a light flux comingfrom the coupling lens 2 on information recording surface 5 of aninformation recording medium, and photo-detector 4 which receivesreflected light from information recording surface 5 of an informationrecording medium are provided.

The optical pickup apparatus in FIG. 8 is further provided with beamsplitter 6 which splits the reflected light from information recordingsurface 5 toward the photo-detector 4, ¼ wavelength plate 7 locatedbetween coupling lens 2 and objective lens 1, diaphragm 8 located to beahead. of objective lens 8, cylindrical lens 9 and actuator 10 for focustracking. In other words, in the present embodiment, the lightconverging optical system has therein a beam splitter, a coupling lens,a ¼ wavelength plate, an objective lens and a diaphragm. Incidentally,in the present embodiment, the beam splitter may be regarded as onewhich is not included in a light converging optical system.

Objective lens 1 further has, on its outer circumference, flange portionla having a surface extending in the direction perpendicular to theoptical axis. Due to this flange portion la, objective lens 1 can beattached accurately on the optical pickup apparatus.

The coupling lens 2 may also be a collimating lens which collimates anincident divergent light flux to be a light flux that is almost inparallel with an optical axis. In this case, it is preferable that lightsource 3 or collimating lens 2 is arranged to be movable in thedirection of an optical axis of the collimating lens for adjustment sothat a light flux emerging from the collimating lens 2 may nearly becollimated.

As stated above, the optical pickup apparatus of the invention mayeither be composed of a collimating lens for converting a divergentlight flux from a light source into a mostly collimated light and of anobjective lens for converging the collimated light on an informationrecording surface, or be composed of a coupling lens representing aconverting lens which changes an angle of a divergent light flux from alight source and converts into a diverged light flux or a convergedlight flux and of an objective lens which converges a light fluxemerging from the coupling lens on an information recording surface. Theoptical pickup apparatus may further be composed only of an objectivelens (finite conjugational objective lens) for converging a divergentlight flux from a light source on an information recording surface.

Then, it is possible to obtain a optical pickup apparatus capable ofconducting high density recording and reproducing for an optical disk,by using an aspherical single objective lens of the invention for theaforesaid optical pickup apparatus.

EXAMPLE

Next, there will be explained Examples 1-15 each being for an objectivelens and a optical pickup apparatus both of the invention and Examples16-27 each being for a coupling lens and a coupling lens and a opticalpickup apparatus. Incidentally, an example of the schematic structure ofthe optical pickup apparatus is like what is shown in FIG. 8 explainedin an embodiment. The optical pickup apparatus of the invention wasobtained by conducting selection of the standard wavelength of asemiconductor laser (setting of a light source), establishment such asusage or elimination of a coupling lens or usage of a collimating lensas a coupling lens, establishment of an aperture of diaphragm 8 andestablishment of positions for arranging various parts, and by mountingan objective lens and a coupling lens of each example, so that thestructures and conditions described in each example below may besatisfied.

First, an example of an objective lens will be explained. A list of dataof Examples 1-15 is shown in Table 1 below. Incidentally, in Examples1-15, Examples 6, 9 and 15 are for a plastic lens, and others are for aglass lens. An optical information recording medium in Example 5 has notransparent substrate. Each of optical information recording media inother Examples has a 0.1 mm-thick transparent

TABLE 1 List of Examples 1 2 3 4 5 f 1.76 1.76 1.76 1.76 0.13 NA 0.850.75 0.85 0.75 0.83 Wavelength 400 400 400 400 660 (nm) d1/f 1.79 1.731.68 1.59 1.74 f/νd 0.048 0.048 0.033 0.033 0.003 νd 37.0 37.0 53.2 53.240.9 r1/(n · f) 0.53 0.53 0.50 0.50 0.49 n 1.85614 1.85614 1.716671.71667 1.79998 Wave Axial 0.010 0.005 0.012 0.006 0.003 front off-0.054 0.029 0.060 0.033 0.019 aberra- axis tion (λ (Image (0.03 (0.03(0.03 mm) (0.03 mm) (0.005 rms) height) mm) mm) mm) (Angle (1°) (1°)(1°) (1°) (2°) of view) List of Examples 6 7 8 9 10 f 2.65 1.76 1.761.76 1.76 NA 0.85 0.85 0.85 0.85 0.85 Wavelength 400 400 405 405 405(nm) d1/f 1.79 1.42 1.56 1.47 1.51 f/νd 0.047 0.048 0.033 0.030 0.019 νd56.0 37.0 53.2 59.5 95.0 r1/(n · f) 0.48 0.47 0.47 0.44 0.42 n 1.561191.85614 1.71558 1.52523 1.44260 Wave Axial 0.022 0.005 0.008 0.008 0.014front off- 0.121 0.070 0.063 0.098 0.118 aberra- axis tion (λ (Image(0.03 (0.03 (0.03 mm) (0.03 mm) (0.03 mm) rms) height) mm) mm) (Angle(0.6°) (1°) (1°) (1°) (1°) of view) List of Examples 11 12 13 14 15 f1.76 1.76 1.76 1.76 1.76 NA 0.85 0.85 0.85 0.85 0.85 Wavelength 405 405405 405 405 (nm) d1/f 1.50 1.36 2.07 12.22 1.43 f/νd 0.022 0.083 0.0830.106 0.030 νd 81.6 21.2 21.2 16.6 59.5 r1/(n · f) 0.44 0.44 0.60 0.640.46 n 1.50716 2.15857 2.15857 2.34860 1.52523 Wave Axial 0.009 0.0020.006 0.006 0.010 front Off- 0.106 0.112 0.032 0.030 0.081 aberra- axistion (λ (Image (0.03 (0.03 (0.03 mm) (0.03 mm) (0.03 mm) rms) height)mm) mm) (Angle (1°) (1°) (1#) (1°) (1°) of view)

An objective lens in Examples 1-4 and Examples 6 and 7 is an infiniteobjective lens for the standard wavelength of 400 nm, and an objectivelens in Examples 8-15 and Examples 6 and 7 is an infinite objective lensfor the standard wavelength of 405 nm. In each of Examples 6 and 9, aprotective layer of an information recording medium having a thicknessof 0.1 mm is assumed to be positioned and a working distance of not lessthan 0.1 mm is provided between an objective lens and an image surfaceof an information recording medium, and plastic materials are used forthe objective lens. An objective lens in Example 5 is an infiniteobjective lens for the standard wavelength of 660 nm.

Example 15 is one wherein a diffraction section is provided.Incidentally, the term described as “Off-axis” of “Wave frontaberration” in Table 1 shows image height characteristics. Table 1 showsthat image height characteristics in Example 1-Example 15 are excellent.In Example 8, wave front aberration caused by the first surface that isdecentered by 1 μm is 0.021 λ which makes eccentricity sensitivity to beexcellent because it is smaller than 0.035 λ. It was possible to makeeccentricity sensitivity to be excellent even in other Examples.

With regard to an aspherical surface in the present example, it isexpressed by the following expression when an x-axis is represented bythe direction of an optical axis, a height in the directionperpendicular to the optical axis is represented by h, and a radius ofcurvature of the surface is represented by r, on the assumption that Krepresents a constant of the cone and A_(2i) represents an asphericalsurface coefficient.$x = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + K} \right){h^{2}/r^{2}}}}} + {\sum\limits_{i = 2}\quad {A_{2i}h^{2i}}}}$

Example 1

Lens data are shown in Table 2, and aspherical surface coefficients areshown in Table 3. The lens of Example 1 is shown in FIG. 1 wherein FIG.1(a) is a sectional view and FIG. 1(b) is an aberration diagram.

λ (wavelength)=400 nm

f=1.765 mm

NA=0.85

Magnification=0

TABLE 2 r(mm) d(mm) n νd 1*   1.72078 3.150 1.85614 37.0 2* −1.927530.213 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 3 Aspherical surface coefficient First surface Second surface  K =−0.319957  K = −126.71803  A₄ = −0.897201 × 10⁻³  A₄ = 0.446627 × 1  A₆= −0.132966 × 10⁻²  A₆ = −0.374370 × 10  A₈ = 0.567005 × 10⁻³  A₈ =0.128630 × 10 A₁₀ = −0.488314 × 10⁻³ A₁₀ = −0.176551 × 10 A₁₂ = 0.337127× 10⁻⁴ A₁₂ = 0.252229 × 10⁻³ A₁₄ = 0.426690 × 10⁻⁴ A₁₆ = −0.200712 ×10⁻⁴

Example 2

Lens data are shown in Table 4, and aspherical surface coefficients areshown in Table 5. The lens of Example 2 is shown in FIG. 2 wherein FIG.2(a) is a sectional view and FIG. 2(b) is an aberration diagram.

λ (wavelength)=400 nm

f=1.765 mm

NA=0.75

Magnification=0

TABLE 4 r(mm) d(mm) n νd 1*   1.72793 3.037 1.85614 37.0 2* −2.276460.272 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 5 Aspherical surface coefficient First surface Second surface  K =−0.332121  K = −87.525272  A₄ = −0.142338 × 10⁻²  A₄ = 0.378863 × 1  A₆= −0.145971 × 10⁻²  A₆ = −0.330567 × 10  A₈ = 0.480431 × 10⁻³  A₈ =0.125735 × 10² A₁₀ = −0.506544 × 10⁻³ A₁₀= −0.193685 × 10² A₁₂ =0.213333 × 10⁻⁴ A₁₂= −0.252229 × 10⁻³ A₁₄ = 0.180460 × 10⁻⁴ A₁₆ =−0.104472 × 10⁻⁴

Example 3

Lens data are shown in Table 6, and aspherical surface coefficients areshown in Table 7. The lens of Example 3 is shown in FIG. 3 wherein FIG.3(a) is a sectional view and FIG. 3(b) is an aberration diagram.

λ (wavelength)=400 nm

f=1.765 mm

NA=0.85

Magnification=0

TABLE 6 r(mm) d(mm) n νd 1*   1.51143 2.946 1.71667 53.2 2* −1.444150.267 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 7 Aspherical surface coefficient First surface Second surface  K =−0.435901  K = −59.503252  A₄ = 0.227660 × 10⁻²  A₄ = 0.330895 × 1  A₆ =−0.331034 × 10⁻²  A₆ = −0.173954 × 10  A₈ = 0.363944 × 10⁻²  A₈ =0.376531 × 10 A₁₀ = −0.258170 × 10⁻² A₁₀ = −0.327613 × 10 A₁₂ = 0.676932× 10⁻³ A₁₂ = −0.252229 × 10⁻³ A₁₄ = 0.153229 × 10⁻⁴ A₁₆ = −0.463776 ×10⁻⁴

Example 4

Lens data are shown in Table 8, and aspherical surface coefficients areshown in Table 9. The lens of Example 4 is shown in FIG. 4 wherein FIG.4(a) is a sectional view and FIG. 4(b) is an aberration diagram.

λ (wavelength)=400 nm

f=1.765 mm

NA=0.75

Magnification=0

TABLE 8 r(mm) d(mm) n νd 1*   1.51629 2.801 1.71667 53.2 2* −1.744960.342 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 9 Aspherical surface coefficient First surface Second surface  K =−0.448813  K = −46.678777  A₄ = 0.580310 × 10⁻³  A₄ = 0.219283 × 1  A₆ =−0.158678 × 10⁻²  A₆ = −0.124381 × 10  A₈ = 0.136862 × 10⁻²  A₈ =0.291780 × 10 A₁₀ = −0.198562 × 10⁻² A₁₀ = −0.280227 × 10 A₁₂ = 0.114053× 10⁻² A₁₂ = −0.252229 × 10⁻³ A₁₄ = −0.438727 × 10⁻³ A₁₆ = 0.508367 ×10⁻⁴

Example 5

Lens data are shown in Table 10, and aspherical surface coefficients areshown in Table 11. The lens of Example 5 is shown in FIG. 5 wherein FIG.5(a) is a sectional view and FIG. 5(b) is an aberration diagram.

λ (wavelength)=660 nm

f=0.131 mm

NA=0.83

Magnification=−0.1456

TABLE 10 r(mm) d(mm) n νd 1* 0.115 0.226 1.79998 40.9 2* −0.147 0.000 *:Aspherical surface

TABLE 11 Aspherical surface coefficient First surface Second surface  K= −0.3946  K= −77.181  A₄ = −0.78479 × 10  A₄ = 0.24008 × 10²  A₆ =−0.23519 × 10⁴  A₆ = −0.10585 × 10⁵  A8 = 0.56266 × 10⁵ _( A) ₈ =0.93242 × 10⁶ A₁₀ = −0.27400 × 10⁷ A₁₂ = −0.10004 × 10¹⁰ A₁₂ = −0.20657× 10⁹ A₁₄ = 0.75407 × 10⁷ A₁₆ = −0.35744 × 10¹¹

Example 6

Lens data are shown in Table 12, and aspherical surface coefficients areshown in Table 13. The lens of Example 6 is shown in FIG. 6 wherein FIG.6(a) is a sectional view and FIG. 6(b) is an aberration diagram.

λ (wavelength)=400 nm

f=2.647 mm

NA=0.85

Magnification=0

TABLE 12 r(mm) d(mm) n νd 1*   1.97771 4.748 1.56119 56.0 2* −0.817680.300 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 13 Aspherical surface coefficient First surface Second surface  K= −0.576418  K = −19.183803  A₄ = 0.265281 × 10⁻²  A₄ = 0.335865 × 1  A₆= −0.413751 × 10⁻³  A₆ = −0.922525 × 1  A₈ = 0.317393 × 10⁻³  A₈ =0.116730 × 10 A₁₀ = −0.591851 × 10⁻⁴ A₁₀ = −0.591738 × 1 A₁₂ = −0.442060× 10⁻⁵ A₁₂ = −0.291540 × 10⁻⁵ A₁₄ = 0.362723 × 10⁻⁵ A₁₆ = −0.412233 ×10⁻⁶

Example 7

Lens data are shown in Table 14, and aspherical surface coefficients areshown in Table 15. The lens of Example 7 is shown in FIG. 7 wherein FIG.7(a) is a sectional view and FIG. 7(b) is an aberration diagram.

λ (wavelength)=400 nm

f=1.765 mm

NA=0.85

Magnification=0

TABLE 14 r(mm) d(mm) n νd 1*    1.53773 2.500 1.85614 37.0 2* −21.608330.380 3  ∞ 0.100 1.62158 30.0 4  ∞ 0.000 *: Aspherical surface

TABLE 15 Aspherical surface coefficient First surface Second surface  K= −0.329489  K = 199.72542  A₄ = −0.168113 × 10⁻²  A₄ = 0.344557 × 1  A₆= −0.913997 × 10⁻³  A₆ = −0.119299 × 10  A₈ = −0.127668 × 10⁻³  A₈ =0.181507 × 10 A₁₀ = −0.319026 × 10⁻³ A₁₀ = −0.110457 × 10 A₁₂ = 0.691773× 10⁻⁴ A₁₂ = −0.252229 × 10⁻³ A₁₄ = −0.241646 × 10⁻⁴ A₁₆ = −0.187683 ×10⁻⁴

Example 8

Lens data and aspherical surface coefficients are shown in Table 16. Asectional view of the objective lens of Example 8 is shown in FIG. 9,and an aberration diagram thereof is shown in FIG. 10.

TABLE 16 Example 8 λ = 405[nm] f = 1.765[mm] Magnification = 0 r(mm)d(mm) n νd 1*   1.43376 2.750 1.71558 53.2 2* −2.11753 0.290 3  ∞ 0.1001.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient First surfaceSecond surface  K = −0.452646  K = −185.75159  A₄ = 0.571669E − 02  A₄ =0.281279E + 00  A₆ = −0.591147E − 02  A₆ = −0.742134E + 00  A₈ =0.721339E − 02  A₈ = 0.667680E + 00 A₁₀ = −0.398819E − 02 A₁₀ =−0.195290E + 00 A₁₂ = 0.390519E − 03 A₁₂ = −0.252228E − 03 A₁₄ =0.446956E − 03 A₁₆ = −0.135385E − 03 *: Aspherical surface

Example 9

Lens data and aspherical surface coefficients are shown in Table 17. Asectional view of the objective lens of Example 9 is shown in FIG. 11,and an aberration diagram thereof is shown in FIG. 12.

TABLE 17 Example 9 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification = 0r(mm) d(mm) n νd 1*   1.17503 2.602 1.52523 59.5 2* −1.04152 0.357 3  ∞0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.682004  K = −29.373780  A₄ = 0.180213E −01  A₄ = 0.297543E + 00  A₆ = 0.368416E − 02  A₆ = −0.423018E + 00  A₈ =0.140365E − 02  A₈ = 0.295535E + 00 A₁₀ = 0.342876E − 03 A₁₀ =−0.829290E − 01 A₁₂ = −0.311534E − 04 A₁₂ = −0.252257E − 03 A₁₄ =0.103341E − 03 A₁₆ = 0.141728E − 04 *: Aspherical surface

Example 10

Lens data and aspherical surface coefficients are shown in Table 18. Asectional view of the objective lens of Example 10 is shown in FIG. 13,and an aberration diagram thereof is shown in FIG. 14.

TABLE 18 Example 10 λ = 405[nm] f = 1.765[nm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1*   1.07547 2.657 1.44260 95.0 2* −0.69088 0.366 3 ∞ 0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.700141  K = −12.891107  A₄ = 0.190496E −01  A₄ = 0.262567E + 00  A₆ = 0.502475E − 02  A₆ = −0.355053E + 00  A₈ =0.115240E − 02  A₈ = 0.236709E + 00 A₁₀ = 0.134395E − 03 A₁₀ =−0.631951E − 01 A₁₂ = 0.369702E − 04 A₁₂ = −0.253345E − 03 A₁₄ =0.315362E − 03 A₁₆ = −0.398715E − 04 *: Aspherical surface

Example 11

Lens data and aspherical surface coefficients are shown in Table 19. Asectional view of the objective lens of Example 11 is shown in FIG. 15,and an aberration diagram thereof is shown in FIG. 16.

TABLE 19 Example 11 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1*   1.15821 2.647 1.50716 81.6 2* −0.90947 0.346 3 ∞ 0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.661186  K = −24.300945  A₄ = 0.159215E −01  A₄ = 0.296712E + 00  A₆ = 0.483822E − 02  A₆ = −0.416550E + 00  A₈ =−0.630221E − 03  A₈ = 0.289015E + 00 A₁₀ = 0.130734E − 02 A₁₀ =−0.807695E − 01 A₁₂ = −0.585454E − 04 A₁₂ = −0.252243E − 03 A₁₄ =−0.503797E − 04 A₁₆ = 0.569157E − 04 *: Aspherical surface

Example 12

Lens data and aspherical surface coefficients are shown in Table 20. Asectional view of the objective lens of Example 12 is shown in FIG. 17,and an aberration diagram thereof is shown in FIG. 18.

TABLE 20 Example 12 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1* 1.69377 2.400 2.15857 21.2 2* 2.36431 0.361 3  ∞0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.413733  K = −9.944350  A₄ = 0.330545E −02  A₄ = 0.834366E − 01  A₆ = −0.226795E − 03  A₆ = −0.534921E + 00  A₈= 0.133470E − 02  A₈ = 0.647444E + 00 A₁₀ = −0.133780E − 02 A₁₀ =−0.195829E + 00 A₁₂ = 0.654514E − 03 A₁₂ = −0.252217E − 03 A₁₄ =−0.152871E − 03 A₁₆ = 0.488831E − 05 *: Aspherical surface

Example 13

Lens data and aspherical surface coefficients are shown in Table 21. Asectional view of the objective lens of Example 13 is shown in FIG. 19,and an aberration diagram thereof is shown in FIG. 20.

TABLE 21 Example 13 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1*   2.30000 3.650 2.15857 21.2 2* −2.73024 0.200 3 ∞ 0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.456357  K = −219.93144  A₄ = 0.712833E −03  A₄ = −0.962060E − 01  A₆ = −0.604365E − 03  A₆ = −0.200434E + 00  A₈= 0.898662E − 03  A₈ = 0.741851E + 00 A₁₀ = −0.133726E − 02 A₁₀ =−0.292121E + 00 A₁₂ = 0.785181E − 03 A₁₂ = −0.252226E − 03 A₁₄ =−0.223083E − 03 A₁₆ = 0.199958E − 04 *: Aspherical surface

Example 14

Lens data and aspherical surface coefficients are shown in Table 22. Asectional view of the objective lens of Example 14 is shown in FIG. 21,and an aberration diagram thereof is shown in FIG. 22.

TABLE 22 Example 14 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1*   2.64228 3.919 2.34860 16.6 2* −3.55612 0.200 3 ∞ 0.100 1.61950 30.0 4  ∞ 0.000 Aspherical surface coefficient Firstsurface Second surface  K = −0.467576  K = −321.19491  A₄ = 0.555792E −03  A₄ = −0.195720E + 00  A₆ = −0.149475E − 02  A₆ = 0.310925E + 00  A₈= 0.178501E − 02  A₈ = −0.244958E + 00 A₁₀ = −0.157718E − 02 A₁₀ =0.486778E − 01 A₁₂ = 0.651169E − 03 A₁₂ = −0.252220E − 03 A₁₄ =−0.127250E − 03 A₁₆ = 0.484981E − 05 *: Aspherical surface

Example 15

Lens data and aspherical surface coefficients are shown in Table 23. Asectional view of the objective lens of Example 15 is shown in FIG. 23,and an aberration diagram thereof is shown in FIG. 24.

TABLE 23 Example 15 λ = 405[nm] f = 1.765[mm] NA = 0.85 Magnification =0 r(mm) d(mm) n νd 1 (Aspherical surface 1,  1.23647 2.532 1.52523 59.5Diffraction surface 1) 2 (Aspherical surface 2) −1.18419 0.336 3 ∞ 0.1001.61950 30.0 4 ∞ 0.000 Aspherical surface 1 Diffraction surface 1 K =−0.68816 b₂ = −0.20985E−01 A₄ = 0.17621E−01 b₄ = −0.26478E−02 A₆ =0.32160E−02 b₆ = −0.31346E−03 A₈ = 0.17762E−02 b₈ = −0.63327E−04 A₁₀ =0.28747E−03 b₁₀ = −0.45002E−04 A₁₂ = −0.17669E−03 b₁₂ = −0.20458E−04 A₁₄= 0.94949E−04 b₁₄ = −0.10510E−04 A₁₆ = 0.17955E−04 b₁₆ = 0.36615E−05Aspherical surface 2 K = −41.704463 A₄ = 0.362699E+00 A₆ = −0.534069E+00A₈ = 0.354745E+00 A₁₀ = −0.793612E−01 A₁₂ = −0.252257E−03

Incidentally, the diffraction surface can be expressed by the followingexpression as optical path difference function Φb, (which also appliesto Example 26 explained later). In this case, h represents a height inthe direction perpendicular to an optical axis, and b represents acoefficient of the optical path difference function.$\Phi_{b} = {\sum\limits_{i = 1}^{\infty}\quad {b_{2i}h^{2i}}}$

As stated above, in Examples 1-15, it was possible to obtain anaspherical single objective lens having a large numerical aperture andexcellent image height characteristics as an objective lens for aoptical pickup apparatus. For example, it was possible to obtain asingle objective lens wherein the numerical aperture is 0.85 forwavelength of 400 nm, and rms of the wave front aberration is not morethan 0.07 λ (λ is a wavelength) for an image height at an angle of viewof 1°, as shown in Example 1. Namely, it was possible to obtain anaspherical single objective lens for a optical pickup apparatus whichhas a large numerical aperture of 0.65 or more and excellent imageheight characteristics and is suitable for a high density recording andreproducing apparatus.

Further, in Examples 1-15, eccentricity sensitivity can be madeexcellent and spherical aberration and coma can be correctedsatisfactorily.

Next, an example of a coupling lens will be explained. Table 24 belowshows a list of data for Examples 16-27. Incidentally, objective lensesin Examples 16, 17, 20, 21 and 22 are the same as that in Example 8,objective lenses in Examples 18, 19, 23, 24, 25 and 26 are the same asthat in Example 9, and an objective lens in Example 27 is the same asthat in Example 13.

TABLE 24 Example 16 17 18 19 20 21 Material Glass Glass Plastic PlasticGlass Glass of an objective lens Focal 1.765 1.765 1.765 1.765 1.7651.765 length of an objective lens NA of an 0.85 0.85 0.85 0.85 0.85 0.85objective lens Standard 405 nm 405 nm 405 nm 405 nm 405 nm 405 nm wave-length Structure Spherical Spherical Spherical Spherical Aspheri-Aspheri- of a doublet doublet doublet doublet cal cal coupling doubletdoublet lens Divergent Collima- Collima- Collima- Collima- Collima-Collima- angle of ted light ted light ted light ted light ted light tedlight a light flux flux flux flux flux flux flux emerging from acoupling lens 0.1 ≦ |m| ≦ 0.20 0.13 0.20 0.13 0.33 0.20 0.5 (m < 0) δfb· NA² ≦ 0.14 0.087 0.16 0.12 0.071 0.034 0.25 μm 0.02 μm ≦ δfb · NA² ≦0.15 μm (δfb) 0.19 0.12 0.22 0.17 0.098 0.047 Example 22 23 24 25 26 27Material Glass Plastic Plastic Plastic Plastic Glass of an havingobjective high lens refrac- tive index Focal 1.765 1.765 1.765 1.7651.765 1.765 length of an objective lens NA of an 0.85 0.85 0.85 0.850.85 0.85 objective lens Standard 405 nm 405 nm 405 nm 405 nm 405 nm 405nm wave- length Structure Aspheri- Aspheri- Aspheri- Aspheri- SingleAspheri- of a cal cal cal cal diffrac- cal coupling doublet doubletdoublet doublet tion lens doublet lens Divergent Collima- Collima-Collima- Collima- Collima- Collima- angle of ted light ted light tedlight ted light ted light ted light a light flux flux flux flux fluxflux flux emerging from a coupling lens 0.1 ≦ |m| ≦ 0.13 0.33 0.20 0.130.29 0.10 0.5 (m < 0) δfb · NA² ≦ 0.0031 0.10 0.060 0.031 0.12 0.06 0.25μm 0.02 μm ≦ δfb · NA² ≦ 0.15 μm (δfb) 0.0043 0.14 0.083 0.043 0.17 0.08

Example 16

Lens data and an aspherical surface coefficient are shown in Table 25.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 16 and an objective lens are shown in FIG. 25, anda diagram of spherical aberration is shown in FIG. 26.

TABLE 25 Surface No. r(mm) d(mm) n νd Light source 6.410 1 Coupling−65.708  1.423 1.91409 23.8 2 lens  5.042 2.242 1.75166 54.7 3 −5.0335.000 Diaphragm ∞ 0    4(Aspherical Objective  1.434 2.750 1.71558 53.2surface 1) lens 5(Aspherical −2.118 0.290 surface 2) 6 Transparent ∞0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 Asphericalsurface 2 K = −0.452646 K = −185.751580 A₄ = 0.571669E−2 A₄ = 0.281279A₆ = −0.591147E−2 A₆ = −0.742134 A₈ = 0.721339E−2 A₈ = 0.667680 A₁₀ =−0.398819E−2 A₁₀ = −0.195290 A₁₂ = 0.390519E−3 A₁₂ = −0.252228E−3 A₁₄ =0.446956E−3 A₁₆ = −0.135385E−3

Example 17

Lens data and an aspherical surface coefficient are shown in Table 26.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 17 and an objective lens are shown in FIG. 27, anda diagram of spherical aberration is shown in FIG. 28.

TABLE 26 Surface No. r(mm) d(mm) n νd Light source 9.838 1 Coupling−9.865 1.149 1.91409 23.8 2 lens  5.102 2.500 1.75166 54.7 3 −4.7135.000 Diaphragm ∞ 0    4(Aspherical Objective  1.434 2.750 1.71558 53.2surface 1) lens 5(Aspherical −2.118 0.290 surface 2) 6 Transparent ∞0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 Asphericalsurface 2 K = −0.452646 K = −185.751580 A₄ = 0.571669E−2 A₄ = 0.281279A₆ = −0.591147E−2 A₆ = −0.742134 A₈ = 0.721339E−2 A₈ = 0.667680 A₁₀ =−0.398819E−2 A₁₀ = −0.195290 A₁₂ = 0.390519E−3 A₁₂ = −0.252228E−3 A₁₄ =0.446956E−3 A₁₆ = −0.135385E−3

Example 18

Lens data and an aspherical surface coefficient are shown in Table 27.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 18 and an objective lens are shown in FIG. 29, anda diagram of spherical aberration is shown in FIG. 30.

TABLE 27 Surface No. r(mm) d(mm) n νd Light source 6.410 1 Coupling−65.708  1.423 1.91409 23.8 2 lens  5.042 2.242 1.75166 54.7 3 −5.0335.000 Diaphragm ∞ 0    4(Aspherical Objective  1.175 2.602 1.52523 59.5surface 1) lens 5(Aspherical −1.042 0.357 surface 2) 6 Transparent ∞0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 Asphericalsurface 2 K = −0.682004 K = −29.373780 A₄ = 0.180213E−1 A₄ = 0.297543 A₆= 0.368416E−2 A₆ = −0.423018 A₈ = 0.140365E−2 A₈ = 0.295535 A₁₀ =0.342876E−3 A₁₀ = −0.829290E−1 A₁₂ = −0.311534E−4 A₁₂ = −0.252257E−3 A₁₄= 0.103341E−3 A₁₆ = 0.141728E−4

Example 19

Lens data and an aspherical surface coefficient are shown in Table 28.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 19 and an objective lens are shown in FIG. 31, anda diagram of spherical aberration is shown in FIG. 32.

TABLE 28 Surface No. r(mm) d(mm) n νd Light source 9.838 1 Coupling−9.865 1.149 1.91409 23.8 2 lens  5.102 2.500 1.75166 54.7 3 −4.7135.000 Diaphragm ∞ 0    4(Aspherical Objective  1.175 2.602 1.52523 59.5surface 1) lens 5(Aspherical −1.042 0.357 surface 2) 6 Transparent ∞0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 Asphericalsurface 2 K = −0.682004 K = −29.373780 A₄ = 0.180213E−1 A₄ = 0.297543 A₆= 0.368416E−2 A₆ = −0.423018 A₈ = 0.140365E−2 A₈ = 0.295535 A₁₀ =0.342876E−3 A₁₀ = −0.829290E−1 A₁₂ = −0.311534E−4 A₁₂ = −0.252257E−3 A₁₄= 0.103341E−3 A₁₆ = 0.141728E−4

Example 20

Lens data and an aspherical surface coefficient are shown in Table 29.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 20 and an objective lens are shown in FIG. 33, anda diagram of spherical aberration is shown in FIG. 34.

TABLE 29 Surface No. r(mm) d(mm) n νd Light source 3.342 1 Coupling 9.926 1.600 1.91409 23.8 2 lens  2.024 2.200 1.71548 53.2 3(Aspherical−3.518 5.000 surface 1) Diaphragm ∞ 0    4(Aspherical Objective  1.4342.750 1.71558 53.2 surface 2) lens 5(Aspherical −2.118 0.290 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical AsphericalAspherical surface 1 surface 2 surface 3 K = 0.270078 K = 0.452646 K =−185,751580 A₄ = 0.425585E−3 A₄ = 0.571669E−2 A₄ = 0.281279 A₆ =−0.968014E−3 A₆ = −0.591147E−2 A₆ = −0.742134 A₆ = 0.315494E−3 A₈ =0.721339E−2 A₈ = 0.667680 A₁₀ = −0.970417E−4 A₁₀ = −0.398819E−2 A₁₀ =−0.195290 A₁₂ = 0.390519E−3 A₁₂ = −0.252228E−3 A₁₄ = 0.446956E−3 A₁₆ =−0.135385E−3

Example 21

Lens data and an aspherical surface coefficient are shown in Table 30.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 16 and an objective lens are shown in FIG. 35, anda diagram of spherical aberration is shown in FIG. 36.

TABLE 30 Surface No. r (mm) d (mm) n νd Light source 7.230 1 Coupling13.531 1.000 1.91409 23.8 2 lens  2.551 2.100 1.71548 53.2 3 (Aspherical−5.765 5.000 surface 1) Diaphragm ∞ 0 4 (Aspherical Objective  1.4342.750 1.71558 53.2 surface 2) lens 5 (Aspherical −2.118 0.290 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 K= 0.699858 A₄ = −0.53797E−3 A₆ = −0.352488E−3 A₈ = 0.595790E−4 A₁₀ =−0.152115E−4 Aspherical surface 2 K = −0.452646 A₄= 0.571669E−2 A₆ =−0.591147E−2 A₈ = 0.721339E−2 A₁₀ = −0.398819E−2 A₁₂ = 0.390519E−3 A₁₄ =0.446956E−3 A₁₆ = −0.135385E−3 Aspherical surface 3 K = −185.751580 A₄ =0.281279 A₆ = −0.742134 A₈ = 0.667680 A₁₀ = −0.195290 A₁₂ = −0.25228E−3

Example 22

Lens data and an aspherical surface coefficient are shown in Table 31.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 22 and an objective lens are shown in FIG. 37, anda diagram of spherical aberration is shown in FIG. 38.

TABLE 31 Surface No. r (mm) d (mm) n νd Light source 11.961 1 Coupling37.967 0.900 1.91409 23.8 2 lens  2.857 2.000 1.71548 53.2 3 (Aspherical−6.448 5.000 surface 1) Diaphragm ∞ 0 4 (Aspherical Objective  1.4342.750 1.71558 53.2 surface 2) lens 5 (Aspherical −2.118 0.290 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 K= 0.980965 A₄ = −0.719068E−3 A₆ = −0.177543E−3 A₈ = 0.364218E−4 A₁₀ =−0.120077E−4 Aspherical surface 2 K = −0.452646 A₄ = 0.571669E−2 A₆ =−0.591147E−2 A₈ = 0.721339E−2 A₁₀ = −0.398819E−2 A₁₂ = 0.390519E−3 A₁₄ =0.446956E−3 A₁₆ = −0.135385E−3 Aspherical surface 3 K = −185.751580 A₄ =0.281279 A₆ = −0.742134 A₈ = 0.667680 A₁₀ = −0.195290 A₁₂ = −0.25228E−3

Example 23

Lens data and an aspherical surface coefficient are shown in Table 32.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 23 and an objective lens are shown in FIG. 39, anda diagram of spherical aberration is shown in FIG. 40.

TABLE 32 Surface No. r (mm) d (mm) n νd Light source 3.342 1 Coupling 9.926 1.600 1.91409 23.8 2 lens  2.024 2.200 1.71548 53.2 3 (Aspherical−3.518 5.000 surface 1) Diaphragm ∞ 0 4 (Aspherical Objective  1.1752.602 1.5253 59.5 surface 2) lens 5 (Aspherical −1.042 0.357 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 K= 0.270078 A₄ = 0.425585E−3 A₆ = −0.968014E−3 A₈ = 0.315494E−3 A₁₀ =−0.970417E−4 Aspherical surface 2 K = −0.682004 A₄ = 0.180213E−1 A₆ =0.368416E−2 A₈ = 0.140365E−2 A₁₀ = 0.342876E−3 A₁₂ = −0.311534E−4 A₁₄ =0.103341E−3 A₁₆ = 0.141728E−4 Aspherical surface 3 K = −29.373780 A₄ =0.297543 A₆ = −0.423018 A₈ = 0.295535 A₁₀ = −0.829290E−1 A₁₂ =−0.252257E−3

Example 24

Lens data and an aspherical surface coefficient are shown in Table 33.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 24 and an objective lens are shown in FIG. 41, anda diagram of spherical aberration is shown in FIG. 42.

TABLE 33 Surface No. r (mm) d (mm) n νd Light source 7.230 1 Coupling13.531 1.000 1.91409 23.8 2 lens  2.551 2.100 1.71548 53.2 3 (Aspherical−5.765 5.000 surface 1) Diaphragm ∞ 0 4 (Aspherical Objective  1.1752.602 1.52523 59.5 surface 2) lens 5 (Aspherical −1.042 0.357 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 K= 0.699858 A₄ = −0.53797E−3 A₆ = −0.352488E−3 A₈ = 0.595790E−4 A₁₀ =−0.152115E−4 Aspherical surface 2 K = −0.682004 A₄ = 0.180213E−1 A₆ =0.368416E−2 A₈ = 0.140365E−2 A₁₀ = 0.342876E−3 A₁₂ = −0.311534E−4 A₁₄ =0.103341E−3 A₁₆ = 0.141728E−4 Aspherical surface 3 K = −29.373780 A₄ =0.297543 A₆ = −0.423018 A₈ = 0.295535 A₁₀ = −0.829290E−1 A₁₂ =−0.252257E−3

Example 25

Lens data and an aspherical surface coefficient are shown in Table 34.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 25 and an objective lens are shown in FIG. 43, anda diagram of spherical aberration is shown in FIG. 44.

TABLE 34 Surface No. r (mm) d (mm) n νd Light source 11.961 1 Coupling37.967 0.900 1.91409 23.8 2 lens  2.857 2.000 1.71548 53.2 3 (Aspherical−6.448 5.000 surface 1) Diaphragm ∞ 0 4 (Aspherical Objective  1.1752.602 1.52523 59.5 surface 2) lens 5 (Aspherical −1.042 0.357 surface 3)6 Transparent ∞ 0.100 1.61950 30.0 7 base plate ∞ Aspherical surface 1 K= 0.980965 A₄ = −0.719068E−3 A₆ = −0.177543E−3 A₈ = 0.364218E−4 A₁₀ =−0.120077E−4 Aspherical surface 2 K = −0.682004 A₄ = 0.180213E−1 A₆ =0.368416E−2 A₈ = 0.140365E−2 A₁₀ = 0.342876E−3 A₁₂ = −0.311534E−4 A₁₄ =0.103341E−3 A₁₆ = 0.141728E−4 Aspherical surface 3 K = −29.373780 A₄ =0.297543 A₆ = −0.423018 A₉ = 0.295535 A₁₀ = −0.829290E−1 A₁₂ =−0.252257E−3

Example 26

Lens data and an aspherical surface coefficient are shown in Table 35.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 26 and an objective lens are shown in FIG. 45, anda diagram of spherical aberration is shown in FIG. 46.

TABLE 35 Surface No. r (mm) d (mm) n νd Light source 5.103 1 (AsphericalCoupling 15.399 2.000 1.52523 59.5 surface 1) lens 2 (Aspherical−5.37697 5 surface 2, Diffraction surface) Diaphragm 0 3 (AsphericalObjective 1.175 2.602 1.52523 59.5 surface 3) lens 4 (Aspherical −1.0420.357 surface 4) 5 Transparent ∞ 0.1 1.61950 30.0 6 base plate ∞ Imageplane Aspherical surface 1 K = 0 A₄ = −0.418319E−1 A₆ = 0.416634E−1 A₈ =−0.159039E−1 A₁₀ = 0.134507E−2 Aspherical surface 2 K = 0 A₄ =−0.22293E−2 A₆ = −0.44722E−3 A₈ = 0.25384E−2 A₁₀ = −0.46638E−3Diffraction surface b₂ = −0.18000E−1 b₄ = −0.80593E−2 b₆ = 0.62172E−2 b₈= −0.26442E−2 b₁₀ = 0.35943E−3 Aspherical surface 3 K = −0.682004 A₄ =0.180213E−1 A₆ = 0.368416E−2 A₈ = 0.140365E−2 A₁₀ = 0.342876E−3 A₁₂ =−0.311534E−4 A₁₄ = 0.103341E−3 A₁₆ = 0.141728E−4 Aspherical surface 4 K= −29.373780 A₄ = 0.297543 A₆ = −0.423018 A₈ = 0.295535 A₁₀ =−0.829290E−1 A₁₂ = −0.252257E−3

Example 27

Lens data and an aspherical surface coefficient are shown in Table 36.Sectional views of a coupling lens of a one-group and two-elementstructure in Example 27 and an objective lens are shown in FIG. 47, anda diagram of spherical aberration is shown in FIG. 48.

TABLE 36 Surface No. r (mm) d (mm) n νd Light source 9.531 1 Coupling−11.04660 0.800 2.34749 16.6 2 lens 1.55601 2.000 1.71548 53.3 3(Aspherical −1.99414 5.000 surface 1) Diaphragm 0 4 (AsphericalObjective 2.3000 3.650 2.15857 21.1 surface 2) lens 5 (Aspherical−2.73024 0.200 surface 3) 6 Transparent ∞ 0.1 1.61950 30.0 7 base plate∞ Image plane Aspherical surface 1 K = 0.53298 A₄ = 0.342156E−2 A₆ =0.133722E−2 A₈ = −0.414740E−3 A₁₀ = 0.257160E−3 Aspherical surface 2 K =−0.456357 A₄ = 0.712833E−3 A₆ = −0.604365E−3 A₈ = 0.898662E−3 A₁₀ =−0.133726E−2 A₁₂ = 0.785181E−3 A₁₄ = −0.223083E−3 A₁₆ = 0.199958E−4Aspherical surface 3 K = 219.931 A₄ = −0.962060E−1 A₆ = −0.200434 A₈ =0.741851 A₁₀ = −0.292121 A₁₂ = −0.252226E−3

Since the objective lens of the invention is a single objective lens ofa refraction system as stated above, there is caused axial chromaticaberration which is under on the part of a short wavelength. However, itwas possible, in Examples 16-27, to correct the axial chromaticaberration by a coupling lens in the composition system of an objectivelens and a coupling lens. It was possible to reduce the axial chromaticaberration of the objective lens by making the axial chromaticaberration of the coupling lens to be over on the part of the shortwavelength.

Further, in Examples 16-27, image height characteristics are alsoexcellent. In addition, eccentricity sensitivity can be made excellentand spherical aberration and coma can be corrected satisfactorily.

Example 28

Another example of the objective lens will be shown next. Lens data andaspherical surface coefficients are shown in Table 37. A sectional viewof the objective lens in Example 28 is shown in FIG. 49 and anaberration diagram is shown in FIG. 50.

TABLE 37 γ = 405 (nm) f = 1.765 (mm) NA = 0.85 Magnification = 0 r(mm)d(mm) N νd 1*   1.19392 2.650 1.52491 56.5 2* −0.97515 0.355 3 ∞ 0.1001.61950 30.0 4 ∞ 0.000 *Aspherical surface Aspherical surfacecoefficient First surface K = −0.683354 A 4 = 0.162029E−01 A 6 =0.154908E−02 A 8 = 0.289288E−02 A10 = −0.367711E−03 A12 = −0.358222E−03A14 = 0.148419E−03 A16 = 0.119603E−03 A18 = −0.302302E−04 A20 =−0.110520E−04 Second surface K = −21.704418 A 4 = 0.308021E+00 A 6 =−0.639499E+00 A 8 = 0.585364E+00 A10 = −0.215623E+00 A12 = −0.252265E−03

Table 38 shows various values.

TABLE 38 Example 28 f 1.76 NA 0.85 Wavelength (nm) 405 d1/f 1.50 f/νd0.031 νd 56.5 r1 (n · f) 0.44 n 1.52491 Wave front Axial 0.006aberration Off-axis 0.086 (Image height) (0.03 mm) (Angle of view) (1°)

In Example 28, since wave front aberration caused by the first surfacethat is decentered by 1 μm is 0.026 λ (it is preferable to be 0.035 λ orless), eccentricity sensitivity is corrected to be excellent.

In the following Examples 29-32, there are shown examples whereinspherical aberrations fluctuate. Optical pickup apparatus used inExamples 29-32 has uniaxial actuator 11 which is shown in FIG. 57 andshifts a coupling lens in the optical axis direction. An objective lens.in each of Examples 29-32 is the same as that in Example 28.

Example 29

Lens data and aspherical surface coefficients are shown in Table 39. Asectional view of the optical system is shown in FIG. 51 and anaberration diagram is shown in FIG. 52.

TABLE 39 Surface No. r (mm) d (mm) n νd Light source d0 (variable) 1Coupling −62.022 1.200 1.52491 56.5 2 lens  −4.608 d2 (variable)Diaphragm ∞ 0    3 (Aspherical Objective   1.194 2.650 1.52491 56.5surface 2) lens 4 (Aspherical  −0.975 0.355 surface 3) 5 Transparent ∞0.100 1.61950 30.0 6 base board ∞ Aspherical surface 1 K = −2.4335E−01A4 = 2.7143E−03 A6 = −5.6745E−05 A8 = 7.0168E−05 A10 = −1.5659E−05Diffraction surface 1 b2 = 2.0000E−02 b4 = −1.3821E−03 Asphericalsurface 2 K = −0.683354 A4 = 0.162029E−01 A6 = 0.154908E−02 A8 =0.289288E−02 A10 = −0.367711E−03 A12 = −0.358222E−03 A14 = 0.148419E−03A16 = 0.119603E−03 A18 = −0.302302E−04 A20 = −0.110520E−04 Asphericalsurface 3 K = −21.704418 A4 = 0.308021E+00 A6 = −0.639499E+00 A8 =0.585364E+00 A10 = −0.215623E+00 A12 = −0.252265E−03

Table 40 Causes of fluctuation of WFE-rms after spherical aberrationcorrection d0 (mm) d2 (mm) Standard conditions  0.007 λ 6.000 5.000 (λc= 405 nm, Tc = 25° C., tc = 0.1 mm) Wavelength Δλ = +10 nm  0.008 λ5.941 5.059 fluctuation Δλ = −10 nm 0.022λ 6.054 4.946 of LD TemperatureΔT = +30° C. 0.011 λ 5.927 5.073 change ΔT = −30° C. 0.031 λ 6.071 4.929Error of Δt = +0.02 mm 0.004 λ 5.853 5.147 transparent Δt = −0.02 mm0.015 λ 6.152 4.848 base board thickness

In the present example, an objective lens and a coupling lens are madeto be a plastic lens. Further, the coupling lens is made to be a singlediffraction lens, and axial chromatic aberration is correctedexcellently by the simple structure.

Table 40 shows that spherical aberration caused by wavelengthfluctuation of a laser, temperature change and by errors in thickness oftransparent base board is corrected excellently.

Example 30

Lens data and aspherical surface coefficients are shown in Table 41. Asectional view of the optical system is shown in FIG. 53 and anaberration diagram is shown in FIG. 54.

TABLE 41 Surface No. r (mm) d (mm) n νd Light source d0 (variable) 1(Aspherical Coupling −226.959 1.200 1.52491 56.5 surface 1, lensdiffraction surface 1) 2 (Aspherical −6.733 d2 surface 2, (variable)diffraction surface 2) Diaphragm ∞ 0    3 (Aspherical Objective  1.1942.650 1.52491 56.5 surface 3) lens 4 (Aspherical −0.975 0.355 surface 4)5 Transparent ∞ 0.100 1.61950 30.0 6 base board ∞ Aspherical surface 1 K= 0.0 A4 = 1.0245E−02 A6 = 9.6650E−04 A8 = −5.9104E−04 A10 = 8.9735E−05Diffraction surface 1 b2 = −2.2967E−02 b4 = 2.1037E−03 Asphericalsurface 2 K = −4.3181 A4 = 1.5848E−03 A6 = 8.6137E−04 A8 = −2.0117E−04A10 = 1.3168E−05 Diffraction surface 2 b2 = −1.7113E−02 b4 = 8.2815E−04Aspherical surface 3 K = −0.683354 A4 = 0.162029E−01 A6 = 0.154908E−02A8 = −0.289288E−02 A10 = −0.367711E−03 A12 = −0.358222E−03 A14 =0.148419E−03 A16 = 0.119603E−03 A18 = −0.302302E−04 A20 = −0.110520E−04Aspherical surface 4 K = −21.704418 A4 = 0.308021E+00 A6 = 0.639499E+00A8 = 0.585364E+00 A10 = −0.215623E+00 A12 = −0.252265E−03

TABLE 42 Causes of fluctuation of WFE-rms after spherical aberrationcorrection d0 (mm) d2 (mm) Standard conditions 0.008 λ 6.000 5.000 (λc =405 nm, Tc = 25° C., tc = 0.1 mm) Wavelength Δλ = +10 nm 0.009 λ 5.8695.131 fluctuation Δλ = −10 nm 0.010 λ 6.141 4.859 of LD Temperature ΔT =+30° C. 0.006 λ 5.905 5.095 change ΔT = −30° C. 0.025 λ 6.101 4.899Error of Δt = +0.02 mm 0.003 λ 5.867 5.133 transparent Δt = −0.02 mm0.014 λ 6.139 4.861 base board thickness

In the present example, an objective lens and a coupling lens are madeto be a plastic lens. Further, the coupling lens is made to be a singleboth-sided diffraction lens, and deterioration of wave front aberrationcaused by mode hop is prevented.

Table 42 shows that spherical aberration caused by wavelengthfluctuation of a laser, temperature change and by errors in thickness oftransparent base board is corrected excellently.

Example 31

Lens data and aspherical surface coefficients are shown in Table 43. Asectional view of the optical system is shown in FIG. 55 and anaberration diagram is shown in FIG. 56.

TABLE 43 Surface No. r (mm) d (mm) n νd Light source d0 (variable) 1Coupling 13.531 1.000 1.91409 23.8 2 lens  2.551 2.100 1.71548 53.2 3(Aspherical −5.765 d3 surface 1) (variable) Diaphragm ∞ 0    3(Aspherical Objective  1.194 2.650 1.52491 56.5 surface 2) lens 4(Aspherical −0.975 0.355 surface 3) 6 Transparent ∞ 0.100 1.61950 30.0 7base board ∞ Aspherical surface 1 K = 0.699858 A4 = −0.53797E−3 A6 =−0.352488E−3 A8 = 0.595790E−4 A10 = −0.152115E−4 Aspherical surface 2 K=−0.683354 A4 = 0.162029E−01 A6 = 0.154908E−02 A8 = 0.289288E−02 A10 =−0.367711E−03 A12 = −0.358222E−03 A14 = 0.148419E−03 A16 = 0.119603E−03A18 = −0.302302E−04 A20 = −0.110520E−04 Aspherical surface 3 K =−21.704418 A4 = 0.308021E+00 A6 = −0.639499E+00 A8 = 0.585364E+00 A10 =−0.215623E+00 A12 = −0.252265E−03

TABLE 44 Causes of fluctuation of WFE-rms after spherical aberrationcorrection d0 (mm) d3 (mm) Standard conditions 0.008 λ 7.230 5.000 (λc =405 nm, Tc = 25° C., tc = 0.1 mm) Wavelength Δλ = +10 nm 0.008 λ 7.1345.096 fluctuation Δλ = −10 nm 0.019 λ 7.330 4.900 of LD Temperature ΔT =+30° C. 0.015 λ 7.050 5.180 change ΔT = −30° C. 0.027 λ 7.415 4.815Error of Δt = +0.02 mm 0.006 λ 6.987 5.243 transparent Δt = −0.02 mm0.015 λ 7.486 4.744 base board thickness

A plastic lens is used as an objective lens. A coupling lens is made tobe a doublet lens of a one-group and two-element type. Further, thesurface of the lens closer to an optical information recording medium ismade to be an aspherical surface. Due to this, compactness and high NAare attained.

Table 44 shows that spherical aberration caused by wavelengthfluctuation of a laser, temperature change and by errors in thickness oftransparent base board is corrected excellently.

Various parameters in Examples 27-31 are shown in Table 45.

TABLE 45 Example 29 30 31 Material of objective Plastic Plastic Plasticlens Focal length of 1.765 1.765 1.765 objective lens NA of objectivelens 0.85 0.85 0.85 Standard wavelength 405 nm 405 nm 405 nm Structureof coupling Single Single Aspherical lens diffraction diffractionsurface lens lens doulet Divergent angle of light Collimated CollimatedCollimated flux emitted from light flux light flux light flux couplinglens 0.1 ≦ |m| ≦ 0.5 (m < 0) 0.26 0.27 0.2 δfB · NA² ≦ 0.25 μm 0.061−0.061 0.032 0.02 μm ≦ δfB · NA² ≦ −0.15 μm (δfB) 0.084 −0.085 0.044

Example 32

The example wherein a diffraction surface is provided only on one sideof a single coupling lens will be shown, next. Lens data and asphericalsurface coefficients are shown in Table 46.

TABLE 46 Surface No. r (mm) d (mm) n νd Light source d0 (variable) 1Coupling ∞ 1.200 1.52491 56.5 (Diffraction lens surface 1) 2 (Aspherical−16.084 d2 surface 1) (variable) Diaphragm ∞ 0    3 (AsphericalObjective  1.194 2.650 1.52491 56.5 surface 2) lens 4 (Aspherical −0.975 0.355 surface 3) 5 Transparent ∞ 0.100 1.61950 30.0 6 base board∞ Diffraction surface 1 b2 = −2.6023E−02 b4 = −2.1722E−04 Asphericalsurface 1 K = 17.997115 A4 = 0.759036E−03 A6 = −0.311883E−03 A8 =−0.123894E−03 A10 = 0.196179E−04 Aspherical surface 2 K = −0.683354 A4 =0.162029E−01 A6 = 0.154908E−02 A8 = 0.289288E−02 A10 = −0.367711E−03 A12= −0.358222E−03 A14 = 0.148419E−03 A16 = 0.119603E−03 A18 =−0.302302E−04 A20 = −0.110520E−04 Aspherical surface 3 K = −21.704418 A4= 0.308021E+00 A6 = −0.639499E+00 A8 = 0.585364E+00 A10 = −0.215623E+00A12 = −0.252265E−03

TABLE 47 Causes of fluctuation of WFE-rms after spherical aberrationcorrection d0 (mm) d2 (mm) Standard conditions 0.005 λ 11.670 5.000 (λc= 405 nm, Tc = 25° C., tc = 0.1 mm) Wavelength Δλ = +10 nm 0.008 λ11.404 5.266 fluctuation Δλ = −10 nm 0.009 λ 11.960 4.710 of LDTemperature ΔT = +30° C. 0.014 λ 11.373 5.297 change ΔT = −30° C. 0.018λ 11.995 4.676 Error of Δt = +0.02 mm 0.009 λ 11.246 5.424 transparentΔt = −0.02 mm 0.008 λ 12.136 4.534 base board thickness

In the present example, a plastic lens is used for an objective lens andfor a coupling lens. Fluctuations of optimum recording position causedby wavelength shift can be controlled to be small, and deterioration ofwave front aberration caused by mode hop is prevented. In addition,deterioration of wave front aberration caused by surface eccentricity ofthe coupling lens is prevented by providing the diffraction surface onlyon one side of the coupling lens. Further, by providing the diffractionsurface on the surface of the coupling lens closer to the light sourceand by providing on the surface of the coupling lens closer to theobjective lens the aspherical surface on which the refracting powergenerated at a location is weaker when the location is farther from theoptical axis, wave front aberration caused by eccentricity of thecoupling lens and by tracking error is prevented. Table 47 shows thatspherical aberration caused by wavelength fluctuation of a laser,temperature change and by errors in thickness of transparent base boardcan be corrected excellently. Axial chromatic aberration can also becorrected excellently. Various parameters in Examples 32 are shown inTable 48.

TABLE 48 Example 32 Material of objective Plastic lens Focal length of1.765 objective lens NA of objective lens 0.85 Standard wavelength 405nm Structure of coupling Single lens diffraction lens Divergent angle oflight Collimated flux emitted from light flux coupling lens 0.1 ≦ |m| ≦0.5 (m < 0) 0.15 δfB · NA² ≦ 0.25 μm 0.05 0.02 μm ≦ δfB · NA² ≦ 0.15 μm(δfB) −0.069

The invention makes it possible to provide an aspherical singleobjective lens having a large numerical aperture and excellent imageheight characteristics, so that it may be used as an objective lens of aoptical pickup apparatus. In particular, it is possible to provide anobjective lens which has a large numerical aperture of 0.75 or more andis suitably used for a high density recording and reproducing apparatusemploying a laser wherein a wavelength of a light source is as short asabout 400 nm.

Further, eccentricity sensitivity can be made excellent and sphericalaberration and coma can be corrected satisfactorily.

It is also possible to provide an objective lens suitably used for arecording and reproducing apparatus which can operate under conditionsthat a thickness of a protective layer of an information recordingmedium is as thin as about 0.1 mm and a working distance is short.

It is further possible to provide a optical pickup apparatus whichemploys the objective lens stated above.

Further, in a high density and optical recording and reproducingapparatus, it is possible to provide a optical pickup apparatus havingan optical system wherein axial chromatic aberration has been correctedby simple structures. In particular, it is possible to provide a opticalpickup apparatus wherein a numerical aperture on the part of aninformation recording medium is as large as 0.65 or more, and theshortest wavelength of a light source to be used is as small as 500 nmor less.

Disclosed embodiment can be varied by a skilled person without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. An apparatus for recording or reproducinginformation in an optical information recording medium, the apparatuscomprising: an optical pickup device including a light source to emitlight flux; and a converging optical system including an objective lensconsisting of a single lens and a coupling lens optically providedbetween the light source and the objective lens; the objective lensconverging the light flux on an information recording surface of theoptical information recording medium and having a numerical aperture notsmaller than 0.65, and the coupling lens changing a divergent angle ofthe light flux emitted from the light source and shifting in an opticalaxis direction so as to correct fluctuation of spherical aberrationcaused in the converging optical system.
 2. An optical pickup apparatuscomprising: a light source to emit light flux; and a converging opticalsystem including an objective lens consisting of a single lens and acoupling lens optically provided between the light source and theobjective lens; the objective lens converging the light flux on aninformation recording surface of an optical information recording mediumand having a numerical aperture not smaller than 0.65, and the couplinglens changing a divergent angle of the light flux emitted from the lightsource and shifting in an optical axis direction so as to correctfluctuation of spherical aberration caused in the converging opticalsystem.
 3. The optical pickup apparatus of claim 2, wherein thenumerical aperture of the objective lens is not smaller than 0.75. 4.The optical pickup apparatus of claim 2, wherein the coupling lenscorrects the fluctuation of spherical aberration caused on each opticalsurface of the objective lens.
 5. The optical pickup apparatus of claim2, wherein the fluctuation of spherical aberration based on change inoscillation wavelength of the light source is corrected by shifting thecoupling lens in the optical axis direction.
 6. The optical pickupapparatus of claim 2, wherein the fluctuation of spherical aberrationbased on at least one of temperature change and humidity change iscorrected by shifting the coupling lens in the optical axis direction.7. The optical pickup apparatus of claim 2, wherein the opticalinformation recording medium has a protective layer for protecting theinformation recording surface and the fluctuation of sphericalaberration, which is based on change in a thickness of the protectivelayer, is corrected by shifting the coupling lens in the optical axisdirection.
 8. The optical pickup apparatus of claim 2, wherein thecoupling lens is shifted in the optical axis direction so that adistance between the coupling lens and the objective lens is increasedwhen the spherical aberration fluctuates in an over-corrected directioncompared with a predetermined value.
 9. The optical pickup apparatus ofclaim 2, wherein the coupling lens is shifted in the optical axisdirection so that a distance between the coupling lens and the objectivelens is decreased when the spherical aberration fluctuates inunder-corrected direction compared with a predetermined value.
 10. Theoptical pickup apparatus of claim 2, further comprising a shifting unitshifting the coupling lens in the optical axis direction.
 11. Theoptical pickup apparatus of claim 2, wherein the objective lenssatisfies the following formula: 1.1≦d 1/f≦3 where d1 represents axiallens thickness and f represents a focal length of the objective lens.12. The optical pickup apparatus of claim 2, wherein the objective lenshas an aspheric surface at at least one optical surface.
 13. The opticalpickup apparatus of claim 2, wherein the objective lens satisfies thefollowing formula: 1.40≦n F/Vd≦0.060 0.40≦r 1/(n·f)≦0.70 where nrepresents a refractive index at a used wavelength, f represents a focallength, Vd represents Abbe's number of the objective lens and r1represents paraxial radius of curvature of the surface of the objectivelens at the light source side.
 14. The optical pickup apparatus of claim2, wherein the objective lens is a plastic lens.
 15. The optical pickupapparatus of claim 2, wherein the objective lens is a glass lens. 16.The optical pickup apparatus of claim 2, wherein a wavelength of thelight flux is not longer than 500 nm.
 17. The optical pickup apparatusof claim 16, wherein the coupling lens corrects chromatic aberration ofthe objective lens.
 18. The optical pickup apparatus of claim 17,wherein the coupling lens comprises two elements in one group.
 19. Theoptical pickup apparatus of claim 17, wherein the coupling lenscomprises a diffractive structure having a plurality of concentricring-shaped steps formed on at least one optical surface of the couplinglens.
 20. The optical pickup apparatus of claim 2, wherein the couplinglens is a plastic lens.
 21. The optical pickup apparatus of claim 2,wherein the optical information recording medium has a protective layerfor protecting the information recording surface, the protective layerhaving a thickness not greater than 0.2 min.